Caspases play a central role in apoptosis that is essential for tissue development, homeostasis and tumor suppression. In the intrinsic pathway, caspase 9 activation is controlled positively by mitochondrial cytochrome c release and negatively by the inhibitor of apoptosis proteins (lAP's) that are subject to inhibition by SMAC/Diablo. Negative regulation of caspase activation in the extrinsic death receptor pathway is poorly understood. Using a yeast two-hybrid screen we have identified a novel Caspase 8 and 10-Associating and degrading Ring domain Protein, CARP. CARP specifically interacts with the death effector domains of caspase 8 and 10, but not caspase 9, FLIP, or FADD. Introduction of CARP into human cells leads to degradation of caspases 8 and 10, but not caspase 9. CARP does not contain a classical BIR domain found in the lAP's but possesses ubiquitin ligase activity mediated by its ring domain. Human cells exposed to the death ligands Fas or TRAIL rapidly cleave CARP protein, suggesting a model wherein elimination of CARP may be an early step during programmed cell death signaling by the extrinsic pathway. The current proposal has the following goals: Specific Aim #1: To map determinants of ubiquitin ligase activity and downstream function of CARP. Specific Aim #2: To understand regulation and cleavage of CARP. Specific Aim 3: To study the functional consequences of perturbing or blocking CARP expression. These studies should provide novel insights into the cellular control of the extrinsic pathway of apoptosis.